how to revert a merge git code example

Example 1: undo a git merge

git reset --hard HEAD~1
git reset --hard <commit_sha>

Example 2: revert last merge git

git revert -m 1 commit_hash

Example 3: git revert pr merge

git revert -m 1 b76a5f1f5d3b323679e466a1a1d5f93c8828b269 (Commit Number)